Aero propellers, either single rotor or contra-rotating, usually have a means of varying the blade pitch via a pitch control mechanism (PCM), to optimise efficiency of thrust delivery and to reduce noise throughout the flight envelope, to provide reverse thrust, and to be able to feather the blades to control drag and rotor speed in some powerplant failure cases. There are a number of established ways of configuring a PCM, but all feature a source of power, prime mover, mechanism from prime mover to blade, and a failsafe system. The power source can be in the static or rotating field, although it is more common for it to be in the static field to avoid static to rotating control communication issues and for easier line replacement of faulty components. However, where the power source is in the static field, a means of transferring the power to the rotating field(s) is required.
For a static electrical power source the transfer is typically achieved via slip rings. These are used on single propeller assembly turboprop engines. However, they suffer from a high maintenance burden. Further, on an engine having two contra-rotating propeller assemblies, and particularly such an engine where the exhaust is ducted under the propeller blade roots, the slip rings would experience very high operating speeds which would significantly reduce slip ring life. The high speeds result from a need to locate the rings at large radial distances in a non-oily zone, as well as from the high relative speeds caused by contra-rotation. Thus slip rings are not seen as a viable solution for power source transfer in contra-rotating propeller assemblies.
For a static hydraulic power source, the transfer can be achieved by rotating hydraulic couplings. For example, in a single rotor engine arrangement, the propeller assembly may be driven by a hollow propeller shaft. A rotating hydraulic coupling can be provided at one end of the propeller shaft, with hydraulic supply lines running inside the shaft from the coupling to a PCM prime mover (e.g. a hydraulic actuator) adjacent the propeller blades. The propeller shaft, supply lines and prime mover are all in the rotating field. A hydraulic pressure power source, which is in the static field, supplies hydraulic fluid to the coupling, and thence to the supply lines.
However, a fundamental design constraint on a rotating hydraulic coupling is that the product (PV) of static to rotating interface velocity (V) and hydraulic pressure (P) should be kept within limits to maintain seal life, assuming positive sealing is necessary. Since propeller rotational speed is generally predetermined, reducing the diameter of the rotating interface is thus of prime importance. Even in circumstances where some leakage is permissible from the rotating hydraulic coupling, reducing the rotating interface diameter helps to decrease the amount of that leakage.
Turboprop engines, whether having a single propeller assembly or two contra-rotating propeller assemblies, employ a reduction gearbox. As shown schematically in FIG. 1, such a gearbox 1 can be of a step-aside shaft configuration in which a drive shaft 2 extending from the free power turbine 3 of the engine 4 is laterally offset from the propeller shaft 5 of the propeller assembly 6. In this configuration, a small diameter, and hence low PV value and low leakage hydraulic coupling 7 may be located at the rear of the gearbox on the end of the propeller shaft, which is hollow. As described above, supply lines 8 can run along the inside of the propeller shaft to supply a hydraulic actuator 9, which rotates with the propeller assembly, with hydraulic fluid from a static hydraulic pressure power source 10.
Alternatively, as shown schematically in FIG. 2, the gearbox 1 can be of a coaxial epicyclic configuration, in which typically a sun gear of the gearbox is driven by and coaxial with the drive shaft 2 extending from the free power turbine 3 of the engine 4. However, as the axis of the propeller, gearbox and gas generator are coincident, it is more problematic to arrange for a small diameter hydraulic coupling 7 with an acceptably low PV value and low leakage rate because the static part of the coupling is outside the propeller shaft 5 outer diameter.
EP A 1881176 proposes an arrangement for transferring hydraulic power from a static hydraulic power source to the respective hydraulic actuators of a contra-rotating turboprop engine which avoids the need for rotating hydraulic couplings, even though the engine has in-line coaxial epicylic gear assembly. In the arrangement, the hydraulic actuators are statically mounted, and the power transfer between the static and rotating fields is achieved by rolling element thrust bearings and associated transfer rods. More particularly, to transfer power to the propeller blade angle adjustment mechanism of the second propeller assembly which is on the other side of the gear assembly from its hydraulic actuator, a first set of transfer rods extend from rolling element thrust bearings at the statically mounted hydraulic actuator through the carrier of the gear assembly planetary gears. As the carrier rotates with the first propeller assembly, a second set of transfer rods then extend from further rolling element thrust bearings between the two sets of rods to the contra-rotating, blade angle adjustment mechanism of the second propeller assembly.
Although the arrangement of EP A 1881176 avoids the use of rotating hydraulic couplings, it raises other issues such as:
(1) The rotor location bearings of the propeller assemblies have to react varying PCM actuation loads as well as contra-rotating propeller thrust loads. To avoid rotor bearing failures associated with load reversal and skidding, the bearings need to be increased in size to withstand the total increased loading. This adds weight, and can also be difficult to achieve within space constraints.
(2) To provide acceptable gear assembly life, appropriate radial and axial stiffnesses between the sun gear, planetary gear carrier and ring gear of the epicylic gear assembly must be achieved. However, the first set of transfer rods extending through the carrier tends to reduce the gear assembly stiffness.
(3) If the first set transfer rods become deformed, e.g. due to gear assembly malfunction, this may compromise the ability of the PCM to adjust the angles of the propeller blades, and in particular to feather the blades to avoid potentially excessive drag and rotor speed.
Thus, it would be desirable to provide an alternative engine arrangement which facilitates the transfer of power to the hydraulic actuator of a propeller assembly.